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Choppy ride

I recently changed the fork oil on my R80. I had the front forks disassembled for a while before I could get started on them again. When I first put them back together I realized that they were way too soft. They would collapse under my weight. I figured that I must have forgotten something but couldn’t figure out what. I cut two pieces of pvc pipe 1.5 inches and added them to the forks for increased spring stiffness. I also experimented with 1 inch pieces but felt the forks would collapse too much. Now the laden/unladen compression difference in inches is within limits but I feel every ripple in the road. Got any advice?

Be sure your initial sag is right. Put bike on center stand and slip a ziptie around the fork just above the seal. The boot might need to be raised. Then the bike off center stand gently. Get on the bike...ideally have someone hold the bike while you lift your feet. Sag should be about 1/3 of total travel, or maybe 2-1/2 inches. This is adjusted with the spacers as you were doing.

Do you have a fairing? That's the only reason you might want to add spacers. As you add spacers, you may stiffen your fork springs but at the same time you're reducing suspension travel. As was said, disassemble and make sure they're reassembled correctly. Use the correct fork oil in the correct amount and your results should be a good handling '78, given that your rear suspension is also correct. Good luck sorting this out and get back on the road soon. gp

If the OP's forks are collapsing under his own weight, something internal is wrong. Not initial suspension sag or whether there is a fairing mounted. If it were my bike I would not ride it until the suspension was corrected.

Pull up a parts diagram, take each leg apart separately and make sure everything is in place. I've worked on several sets of forks from that generation. There is nothing I know of that would cause the forks to be mechanically softer, i.e., collapse from the rider's weight, after dis- and reassembly except a missing part. If you used a much lighter oil they would move up and down more freely but the rider sag would be unchanged. That said, I'm having trouble thinking of a part whose absence would cause such a dramatic change. Did the bike work well before this? What is the rider weight? If you're under 170, the stock springs should be more than adequate.

Missing ball bearing?

I agree that it is a seal or missing part issue. You don't say how far apart you had the forks, and we assume they are not leaking. There is a small 8mm ball bearing and spring assembly at the bottom of the internal pipe/tube component (the one that is drilled for oil flow), and if omitted it may cause the issues you are experiencing.

A full tear down and reassembly after careful review of the parts diagram is probably in order.

While in there, it is probably worth replacing all of the various bushings and seals, as they are relatively cheap vs the labor involved to get in that far.

I have an R80/7 of the same vintage and was experiencing some very strange fork behavior. Sometimes they were incredibly stiff and difficult to even move, and at other times they were extremely squishy. Turns out it was a combination of worn out springs causing the sogginess (these were replaced with a set of Progressive springs and spacers), and a deteriorating bottom buffer ring that was being sucked up in pieces and sometimes clogging the holes of the lower internal drilled tube, which was causing the stiffness.

I think I need to take it apart again as suggested and post pictures then. Is it necessary to use the BMW oil or is there a more convenient alternative? Also, do you guys find that the rear suspension needs to be on the firmest setting?

I think I need to take it apart again as suggested and post pictures then. Is it necessary to use the BMW oil or is there a more convenient alternative? Also, do you guys find that the rear suspension needs to be on the firmest setting?

Good plan ! Almost any quality fork oil of the proper viscosity will work if you are having a hard time sourcing BMW oil.
Unless you weigh in at a biscuit over 400 pounds, if your rear suspension needs to be on the firmest setting it is time for some new rear shocks.

I think I need to take it apart again as suggested and post pictures then. Is it necessary to use the BMW oil or is there a more convenient alternative? Also, do you guys find that the rear suspension needs to be on the firmest setting?

7.5 - 10w fork oil worked best for me but my airheads were of a different generation. By firmest setting, do you mean the spring preload or are your shock adjustable for damping?